Membranipora membranacea

An Invasive Bryozoan in the Gulf of Maine



Membranipora membranacea: An Invasive Species

Invasive species can become dominant if they are released from predation, outcompete prior residents, or occupy an open niche. The bryozoan Membranipora membranacea was introduced into the Gulf of Maine in 1987 and within two years became the dominant species living on kelps. Encrustation by Membranipora may increase the likelihood of breakage of kelps, possibly leading to large changes in the ecosystem.

I am interested in:

  1. How Membranipora spread so quickly?

  2. Consequences of Membranipora invasion


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(1) How did Membranipora spread so quickly?

Some Possibilities:

Predation

Where Membranipora is native, there tends to be a specialist nudibranch (sea slug) predator that keeps the population in check. In European populations, Polycera quadrilineata prefers Membranipora while Onchidoris muricata is known to prefer another bryozoan, Electra pilosa. Electra, Membranipora, and Onchidoris are all now found in the Gulf of Maine while Polycera is not.

Membranipora membranacea

Onchidoris muricata

Electra pilosa

Bowdoin undergraduate honors student Emily Grason and I tested

1) Will Onchidoris eat Membranipora?
2) Does Onchidoris eat Electra or Membranipora faster?
3) Does Onchidoris prefer Electra or Membranipora?

Experimental Design

Onchidoris is a suctorial feeder, which means it eats the polypide without destroying the entire zooid. Thus, it is possible to assess Onchidoris feeding by measuring polypide mortality.We conducted "long term" experiments where we counted the number of live polypides at 0, 4, and 11 days.

We also videotaped the slug eating at 1min intervals over 24h at 15-16ºC to estimate a more accurate feeding rate for Onchidoris.

 

Results

1) Onchidoris will eat Membranipora

 

2) Onchidoris sometimes eats Membranipora faster

When not given a choice between bryozoan species:
Onchidoris ate Membranipora faster in the first 4 days,
but ate Electra faster in the next 7 days.

In the video, Onchidoris ate Membranipora
polypides significantly faster.

 

3) When given a choice, Onchidoris prefers Electra

 

While this experiment shows that at least one native predator will eat Membranipora, the slug prefers another resident bryozoan when given a choice. Thus, one reason why Membranipora was able to spread rapidly in the Gulf of Maine was probably because it was essentially released from predation upon arrival.

 

Available Resources: Open Niche

Perhaps another reason Membranipora was able to spread so quickly was because there were plenty of resources available (open niche). Before Membranipora invaded the Gulf of Maine, not that many organisms lived on kelps so it is possible that Membranipora was able to establish and spread quickly because there was plenty of open substratum to settle and grow on.

I measured the abundance and distribution of Membranipora and Electra on four different species of seaweed (shown below) in the low intertidal/upper subtidal over the course of the summer (2004).

seaweeds

Results

Settlement of new colonies of Electra and Membranipora did not occur until mid-July.  Thus, I assumed that colonies present in early June had survived the winter.

Electra does best on Chondrus:  Electra survived on 50% of the Chondrus plants over winter and by September, Electra had the highest percent cover on Chondrus. Membranipora does not appear to settle on Chondrus in the field, maybe because there is no space available during settlement.

Electra and Membranipora both survive the winter on Palmaria: in June, 50% of the Palmaria examined had Electra present and 25% had Membranipora present. When present, each bryozoan covers ~5% after the winter, which means that there were more small Electra and fewer larger Membranipora on Palmaria in June. Membranipora grows faster over the summer than Electra (by September Membranipora has 2.5 times more percent cover than Electra).

Neither bryozoan survived the winter on either of the brown seaweeds (Fucus and Laminaria).  This is not surprising since Fucus tends to be found a little higher in the intertidal and is exposed to the air, and thus to freezing temperatures, on more tides during the winter.  Laminaria blades do not survive the winter well; new blades are grown each spring from the holdfasts that survive the winter. New settlement of Membranipora and Electra occurred in mid-July: by August, 100% of Laminaria and 88% of the Fucus had Membranipora present, but only 15% of the Laminaria and 42% of the Fucus had Electra present.  By the end of the summer, Membranipora covered 77% of the Laminaria and 50% of the Fucus, while Electra only covered 7% of the Laminaria and 55% of the Fucus.  Membranipora had much higher settlement and the colonies probably grew faster.

Percent Present

Percent Cover  

Overall, these data show that Electra has higher winter survival, but that Membranipora dominates in the late summer and early fall.  The other interesting point to note is that Membranipora does not settle on the algae where Electra has high winter survival and covers a large percentage of the algal blades (Chondrus), but Membranipora has very high settlement on algae where there is no previous residents (Fucus and Laminaria).  In addition, where both bryozoan species are present (either by winter survival or concurrent settlement), Membranipora dominates by the end of the summer/early fall, suggesting that Membranipora has a higher growth rate and outcompetes Electra for space.  I did observe many cases where Membranipora was growing over Electra.  Electra's long term survival may be largely dependent on Chondrus acting as a refugium allowing higher winter survival.

Competition

Where Membranipora and Electra interact, Membranipora tends to take up most of the available attachment space by having much higher settlement and faster growth rates. Click here for a PDF containing part of a recent talk given at the annual meeting for the Society for Integrative and Comparative Biology in Orlando, Florida that gives results comparing growth, feeding, and respiration rates of these two bryozoans.

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(2) Consequences of Membranipora invasion

Predation

Onchidoris typically reproduces in the spring and grows over the winter (as diagrammed below), but has recently been found reproducing in the winter in New Hampshire (L. Harris, University of New Hampshire, personal communication).

While Membranipora is the dominant epiphyte in the late summer and fall, it does not survive the winter as well as Electra. Thus, it will be interesting to see if Onchidoris maintains its current life-history pattern and continues to concentrate on eating Electra, or if the life cycle shifts so that it can take advantage of the large Membranipora food source in the summer and fall.  Because of the huge supply of Membranipora in the late summer and through the fall, if Onchidoris starts to take advantage of Membranipora as a food source this could cause a population explosion of Onchidoris.  Many other indirect effects could result from such a population explosion such as increased predation on Electra and other bryozoans or an equal population explosion of predators of Onchidoris (note: it is currently not known what if anything eats Onchidoris).

Ecosystem Changes

When a population of invasives spreads quickly, large ecosystem changes can result.  The addition of Membranipora may have direct (as discussed above with an increase in the predator population) or indirect effects on the food web.  Food webs in the western North Atlantic generally have four trophic levels, with large predatory fish at the top,  invertebrate predators such as lobsters and crabs at the second highest level, herbivorous sea urchins at the second lowest level, and seaweeds at the bottom.  Evidence gathered by Robert Steneck, from the University of Maine's Darling Marine Center, and colleagues suggests that The Gulf of Maine has gone through three phases:

(1) Phase One: Predatory Fish Dominate
Phase one had stable populations of cod and other predatory fish that kept down the population of lobsters, crabs, and urchins, which allowed lush kelp beds to develop. (lasted from ~4000 years before present to the mid 1960s)

(2) Phase Two: Herbivorous Sea Urchins Dominate
Phase two was characterized by a reduction in predatory fish populations (because of heavy fishing), an expansion of urchins (since their predators had decreased), and a decline in kelps (since the large urchin population was eating all the kelp).  Large aggregations of urchins ate all the seaweeds leaving only a crust of coralline algae (this is called an urchin barren). (lasted from ~1970-1990)

(3) Phase Three: Predatory Invertebrates Dominate
In 1987, intense harvesting of urchins began and kelp beds began to reestablish.  Predatory crustaceans take refuge in the kelp beds and eat any urchins that settle or are reintroduced by humans.  Thus, phase three is dominated by crustaceans and kelp. (began in mid 1990s)


Gulf of Maine Ecosystems

Phase one was stable for a much longer time than phase 2.  While we appear to still be in Phase 3, evidence is mounting that another shift maybe be occurring, which would make phase 3 even shorter than phase 2.  The question is, will Phase 3 continue or will there be another shift?  If there is another shift, what state will it shift to?  
Membranipora appears to decrease kelp growth and survival.  Gaps in kelp beds result from the breakage of kelp blades, and this allows an invasive species of green algae, Codium fragile, to recruit.  Codium is unable to recruit in dense kelp beds because the kelp canopy shades the shorter Codium plants, but once a gap opens up Codium is able to successfully recruit.  Once Codium is established, the kelps are not able to recruit and there is a gradual turnover from kelp to Codium domination.

As a consequence of the Membranipora invasion, there may be shift to a Phase 4 that is dominated by Codium.  The canopy of Codium is still likely to harbor predatory crustaceans.  With predators present and less kelp available, the urchins population is not likely to increase even if pressure from fishing is reduced dramatically.  More research is needed to determine if the ecosystem is really shifting, what is causing the shift, and what state the system is shifing to.


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